NEW YORK — Behind a nondescript glass doorway and up a flight of creaky wooden stairs in the East Village, some of the hottest child stars in the country are crammed into a giant wooden popcorn box.

Smiling or laughing or flashing serious looks on cue, they are posing for a Time magazine cover shot. There's the new Dennis the Menace along with other young stars of this summer's movies, including "Sleepless in Seattle," "Free Willy," "The Secret Garden," "Searching for Bobbie Fisher" and "Life With Mikey."

Standing in a back corner of the box, looking a trifle embarrassed by the proceedings, is Austin O'Brien, co-star of "Last Action Hero," not only one of the season's most anticipated films but, according to some industry observers, possibly the most expensive feature of all time. The new comedy-action-fantasy feature from the many-muscled box office titan Arnold Schwarzenegger opens today on 27 screens in Orange County.

Austin, a 12-year-old from Orange County, plays Danny Madigan, a young action film buff who magically gets pulled into the movie world of his hero Jack Slater, played by Schwarzenegger.

During the movie's five-month shoot, Austin performed many of his own stunts; he got bumped around inside a '66 Bonneville convertible during chase scenes and hung high off the ground from a gargoyle in one climactic sequence. But in some ways, the stunts were a piece of cake compared to the grueling publicity schedule Austin has undertaken in the weeks leading to the film's opening.

This week particularly has seen sandy-haired Austin following a nonstop timetable that matches him almost step-for-step with Schwarzenegger, considered one of Hollywood's most tireless and savvy self-promoters. The Time magazine photo session came in the middle of a nonstop Wednesday that started at 7 a.m. with a limo ride to the studios of "Today" for a taped interview and didn't end until a $300-a-ticket post-screening benefit that didn't start rolling until 10 p.m.

Austin flew back Thursday to Los Angeles where his weeklong blitz continues today, starting with a live interview on "John and Leeza" and climaxing with an appearance on the "Tonight" show with Jay Leno.

Heady stuff for a 12-year-old. But Austin seems to be handling it with a poise beyond his years. For the most part, he says, the attention is "really cool."

"He's remarkable. He's got a very good nature," said Steve Newman, who as the publicity coordinator for "Last Action Hero" is shepherding Austin through his media marathon. The week started Saturday with 41 TV interviews divided only by a short lunch break, followed by an MTV promotional party with Schwarzenegger at Planet Hollywood in Santa Ana.

"It's fun for the first 15 (interviews) or so, then it kind of wipes you out," Austin confessed at the time. "It feels like Dracula sucked all the blood out of me."

That was only the beginning. Sunday he did interviews with domestic print reporters, and attended the movie's world premiere in Westwood. Monday was international press day, followed by a screening for the cast and crew. Tuesday he flew to New York.

Schwarzenegger has made Austin a full partner in the "Last Action Hero" publicity juggernaut since the Cannes Film Festival in May.

"Arnold insisted that Austin be brought along," Newman said. The move paid off in a shot seen 'round the world, with the young actor hoisted upon his co-star's beefy shoulders.

Now that the film is fighting some negative early reviews and rumors of poorly received test screenings, Austin's participation in the publicity continues to pay off in extra air time. Schwarzenegger's "Today" interview was broadcast live; Austin's was taped and shown the next day. Arnold was on the "Tonight" show Thursday; Austin meets Leno tonight.

The New York swing was a typical day for a not-so-typical youngster. In the morning, while Austin's limo carried him back from his "Today" interview, network camera crews were setting up shop in four suites at the St. Regis Hotel. When Austin and Schwarzenegger arrived, they shuttled between the rooms for interviews, like a slow-motion version of a Marx Brothers routine.

Arnold was reported to be in a bit of a bad mood, but he joshed playfully with Austin (What other 12-year-old would get away with calling him "girly man"?). Schwarzenegger's mood might have had to do with the tough questions lobbed at him about the movie's cost and negative buzz, while Austin got the softer queries.

When asked what he would do if the movie got bad reviews, Austin answered with a bit of pure preteen Zen: "I'll just shine it on." For the most part, the questions centered on his relationship with Schwarzenegger, his hobbies and interests (he's a baseball fan), and how it feels to be a star.